The present invention relates to a method for controlling the thickness of a film product.
In extrusion and elongating apparatus for producing a biaxially elongated film product from molten thermoplastic resins, when the extrusion die wears, or is replaced, or when the gap clearance of the die is adjusted for varying the thickness of the film product, or when an elongation rate per unit width of the die needs to be changed, normally, when the production is restarted, the quality of the film product is fairly bad initially, particularly with respect to thickness deviations over the width of the film product.
Thus, an adjustment to remove the thickness deviations needs to be effected after the restarting of the production. However, this adjustment operation is delicate, even if it is done by a skilled worker, and the precision in carrying out such an adjustment is limited. In addition, the working environment close to such apparatus is bad and hence, if the operation is conducted manually, there is a health hazard for the operator. On the other hand, in the case where the adjustment operation for the thickness deviation is effected by an automatic thickness deviation regulating device, the adjustment operations must be completed within a shorter period of time and the final precision with such automatic adjustment must be better than in the case of manual adjustment. However, due to the fact that a reliable and efficient method for determining the correlation between the positions of occurrence of thickness deviations across the width of the film product, and the positions of the manipulation bolts for the extrusion die has not been known heretofore, a satisfactory automatic regulating device is not available and has not been practically used.